Conventionally, indexable rotary cutting tools have been used in 90° shoulder milling operations with the aim of cutting a machined side wall surface of a workpiece at a right angle relative to a machined bottom wall surface. Patent Literatures 1 to 3 each disclose an example of a cutting insert used in the cutting work as described above. The cutting inserts disclosed in these Patent Literatures each have a configuration in which a flank, which continues to a major cutting edge, is divided into three stages in a direction of the central line of the insert. More specifically, the flank, which continues to the major cutting edge, is configured to consist of: a twisted face portion which is a first flank portion, is provided along the major cutting edge and has a relief angle gradually varying along the major cutting edge; a curved surface portion which is a second flank portion and connected with the twisted face portion and a flat surface portion; and a flat surface portion which is a third flank portion and brought into contact with a side wall surface of an insert mounting seat of a tool body.